Not applicable.
Not applicable.
1. Field of the Invention
The present invention relates to offshore mineral drilling and production platforms of the spar type and, more particularly, is concerned with apparatus for supporting drilling and production risers from a gimbaled table supported above the top of the spar hull wherein the table is compliantly constrained, but allowed limited rotational movement with respect to the spar hull.
2. Description of the Prior Art
Drilling and production operations for the exploration and production of offshore minerals require a floating platform that is as stable as possible against environmental forces, even in severe weather conditions. Among the six degrees of freedom of a floating platform, the most troublesome to drilling and production operations are the pitch, heave, and roll motions.
Present spar type floating platforms typically have drilling and production risers that are supported by means of buoyancy cans attached to each of the individual risers. As the water depth in which a platform will be used increases, the diameter and length of the buoyancy cans must be increased to support the in-water weight of the risers and their contents. Larger diameter buoyancy cans require larger spar center well sizes, which in turn increases the spar hull diameter. Increasing the spar hull diameter and size in turn increases the hydrodynamic environmental loads acting on the spar. A larger size mooring system is then required to withstand the increased environmental loads. The total riser buoyancy can system for deep water spar platforms can become very long and heavy, significantly increasing the fabrication and installation costs.
With present spar platforms having a buoyancy can riser support system, as the spar hull displaces laterally in response to environmental loads, the risers undergo a considerable amount of downward motion, or pull-down, with respect to the spar hull. This amount of riser pull-down increases as the water depth and riser length increases, and requires longer jumper hoses, large clear vertical heights between the top of the hull and the drilling deck, and expensive, large stroke keel joints.
Consequently, a need exists for improved apparatus for supporting drilling and production risers from a spar type floating platform. Preferably, such an improved apparatus will eliminate the need for riser buoyancy cans. It will preferably also reduce the amount of riser pull-down relative to the spar hull as the spar pitches and displaces in response to environmental forces. Such an improved riser support apparatus will also preferably reduce the amount of fixed ballast required, reduce the need for, or length of, riser jumper hoses, and reduce the size and diameter of the spar hull. It will also preferably be less expensive to build, install, and maintain than individual riser buoyancy can systems in present use.
The present invention provides a riser support and tensioning apparatus and method that satisfies the aforementioned needs. According to one aspect of the invention, for a spar type floating platform having risers passing vertically through the center well of a spar hull, the spar hull having a top surface, apparatus is provided for supporting the risers from the spar hull. The apparatus comprises a table disposed above the spar hull top surface and a plurality of non-linear springs associated with the table and the spar hull for permitting rotational movement between the table and the spar hull. The apparatus also comprises means for attaching the upper ends of the risers to the table.
According to another aspect of the invention, for a spar type floating platform having risers passing vertically through the center well of a spar hull, the spar hull having a top surface, apparatus is provided for supporting the risers from the spar hull. The apparatus comprises a table disposed above the spar hull top surface. The table comprises a grid having openings therethrough. The risers pass through respective openings in the table grid. For each riser, at least one riser tensioning hydraulic cylinder is provided, having one end attached to the riser and the opposite end attached to the table, such that the tension in and length of the riser may be adjusted by operation of the riser tensioning hydraulic cylinder. A plurality of elastomeric load pads are disposed between the table and the spar hull for permitting rotational movement therebetween. Larger capacity load pads are located near the center of the table for supporting the majority of the riser tension, and smaller capacity load pads are located near the perimeter of the table for controlling the rotational stiffness of the spar hull.
According to a still farther aspect of the invention, for a spar type floating platform having risers passing vertically through the center well of a spar hull, the spar hull having a top surface, apparatus is provided for supporting the risers from the spar hull. The apparatus comprises a table disposed above the spar hull top surface. The table comprises a grid having openings therethrough. The risers pass through respective openings in the table grid. For each riser, at least one riser tensioning hydraulic cylinder is provided, having one end attached to the riser and the opposite end attached to the table, such that the tension in and length of the riser may be adjusted by operation of the riser tensioning hydraulic cylinder. A plurality of table supporting hydraulic cylinders is disposed between the table and the spar hull for permitting rotational movement therebetween. Each table supporting hydraulic cylinder has a first end pivotally attached to the table and a second end pivotally attached to the spar hull. At least one lateral support shaft has an upper end pivotally attached to the table and a lower end. For each lateral support shaft, at least one guide is attached to the spar hull for slidably receiving the lower end of the lateral support shaft.
According to another aspect of the invention, for a spar type floating platform having risers passing vertically through the center well of a spar hull, the spar hull having a top surface, apparatus is provided for supporting the risers from the spar hull. The apparatus comprises a table disposed above the spar hull top surface. The table comprises a grid having openings therethrough. The risers pass through respective openings in the table grid. For each riser, at least one riser tensioning hydraulic cylinder is provided, having one end attached to the riser and the opposite end attached to the table, such that the tension in and length of the riser may be adjusted by operation of the riser tensioning hydraulic cylinder. A plurality of pedestals is provided, each pedestal having a lower end attached to the spar hull and an upper end higher than the table for hanging the table therefrom. For each pedestal, at least one non-linear spring is associated with the table, the pedestal, and the spar hull for permitting rotational movement between the table and the spar hull.
According to still another aspect of the invention, for a spar type floating platform having risers passing vertically through the center well of a spar hull, apparatus is provided for suspending and tensioning a riser from a surface associated with the spar hull, and for permitting limited rotational movement between the riser and the surface. The apparatus comprises a hydraulic cylinder having one end attached to the riser and the other end attached to the surface. The tension in the riser may be adjusted by operation of the hydraulic cylinder. Means is provided for permitting rotational movement between the riser and the surface.
According to still another aspect of the invention, a method is provided for supporting a riser at a floating spar hull, the spar hull having a top surface. The method comprises the step of connecting a table to the spar hull, wherein the table has a limited range of rotational movement with respect to the spar hull top surface in response to environmental forces acting on the spar hull. The method further comprises the steps of suspending the riser from the table and of tensioning the riser.